Raised Bed Soil Calculator: How Much Soil Do You Need?

Most raised beds need far more soil than people expect. To find how much soil your raised bed needs, multiply length by width by depth (all in feet) to get cubic feet, divide by 27 for cubic yards, then add about 15% for settling. A standard 4-by-8-foot bed filled 12 inches deep holds 32 cubic feet, which is roughly 37 one-cubic-foot bags once you add the settling allowance.

We’re Dan and Loan. We grow food for ourselves and our kids in USDA Zone 7b, alongside the eight hens we’ve kept since spring 2026. A soil calculator saves you from two common mistakes: a half-filled bed with no soil left, and a garage full of extra bags.

The math is simple, but the details trip people up. Inside measurements, fill depth, bag size, and settling all change the final number, and so does the blend you choose for your raised garden bed.

Key Takeaways

• Measure inside the frame and convert inches of depth to feet before you multiply.
• Buy about 15% more soil than the raw volume so the bed stays full after it settles.
• Bags make sense for small jobs, and bulk delivery wins once you need about a cubic yard.

Enter your bed’s shape, size, depth, and number of beds in the raised bed soil calculator below to get cubic feet, cubic yards, bag counts, and your topsoil and compost split.

How Much Soil Does Your Bed Need?

Your bed needs a volume of soil equal to its inside area times its fill depth, plus a little extra for settling. Rectangular beds use length times width, round beds use the circle formula, and both convert to cubic yards by dividing by 27.

Enter Your Bed Shape, Inside Dimensions, and Fill Depth

Start by picking rectangle or round. Then measure the inside of the frame, because board thickness eats into the space. A bed sold as 4 by 8 feet built from 2-inch-thick boards might measure closer to 3 feet 8 inches by 7 feet 8 inches inside.

For round beds, measure straight across the middle, inside wall to inside wall. That’s the diameter.

Fill depth is how deep you plan to fill the soil, in inches. It’s often an inch or two less than the frame height, which leaves room for mulch and keeps soil from washing over the edge. If you have several beds of the same size, enter the number of beds and the calculator multiplies for you.

Calculate Cubic Feet and Convert to Cubic Yards

To calculate soil volume by hand, follow these steps:

1. Convert depth from inches to feet by dividing by 12. Six inches is 0.5 feet; 10 inches is about 0.83 feet.
2. For a rectangle, multiply length × width × depth in feet. That’s your soil volume in cubic feet.
3. For a circle, multiply 3.14 × radius × radius × depth. The radius is half the diameter.
4. Divide cubic feet by 27 to get cubic yards, since one cubic yard holds 27 cubic feet.

Bags are labeled in cubic feet. Landscape suppliers sell bulk soil in cubic yards. Knowing both numbers lets you compare them fairly.

Worked Example: a 4-by-8-Foot Bed Filled 12 Inches Deep

This is the most common raised bed size we see.

• Volume: 4 × 8 × 1 = 32 cubic feet
• In cubic yards: 32 ÷ 27 = about 1.19 cubic yards
• With 15% for settling: 32 × 1.15 = about 36.8 cubic feet
• 60% topsoil / 40% compost: about 22 cubic feet topsoil and 15 cubic feet compost

That one bed already needs more than a cubic yard, which affects how you buy it.

Worked Example: a 4-Foot-Diameter Round Bed Filled 10 Inches Deep

Round beds, like galvanized rings, hold less than you’d guess.

• Radius: 4 ÷ 2 = 2 feet
• Depth: 10 ÷ 12 = 0.83 feet
• Volume: 3.14 × 2 × 2 × 0.83 = about 10.5 cubic feet
• In cubic yards: 10.5 ÷ 27 = about 0.39 cubic yards
• With 15% for settling: about 12 cubic feet

Bed	Raw volume	With 15% extra	1 cu ft bags	1.5 cu ft bags	2 cu ft bags
4 × 8 ft, 12 in deep	32 cu ft	36.8 cu ft	37	25	19
4 ft round, 10 in deep	10.5 cu ft	12 cu ft	13	9	7

How Deep Should You Fill Your Bed?

Most vegetable beds do well with 10 to 12 inches of good soil, while greens get by on less and root crops want more. Depth also depends on whether roots can reach the ground below the bed and how much the soil will sink.

Choose Soil Depth for Greens, Vegetables, and Root Crops

Match depth to what you plan to grow:

Crop type	Examples	Suggested soil depth
Shallow-rooted greens and herbs	Lettuce, spinach, radishes, basil	6 to 8 inches
Most vegetables	Tomatoes, peppers, beans, squash	10 to 12 inches
Deep root crops	Carrots, parsnips, potatoes	12 to 18 inches

If you’re planning to start your own herb garden in a raised bed, 6 to 8 inches keeps soil costs low. For a mixed vegetable raised bed garden, 12 inches gives you flexibility from year to year.

Account for Usable Soil Below an Open-Bottom Bed

An open-bottom bed sitting on the ground gets extra root room for free. Beds shallower than 12 inches should be built with no bottom so roots can reach the native soil.

Before filling, loosen the ground under the frame 6 to 12 inches deep with a shovel or broadfork. That loosened layer counts toward root depth, so a 6-inch frame over well-loosened ground can grow most vegetables. If your bed sits on a patio, gravel, or a lined bottom, fill the full depth your crops need.

Why Soil Settles and When to Add 15% Extra

New soil is fluffy and full of air pockets. Watering, rain, and gravity press it down, a process called soil compaction. Compost also breaks down over time, so the level drops through the first season.

A 15% settling allowance covers that loss for most fills. For a 12-inch-deep bed, that’s close to 2 inches of extra soil. Add it every time you fill a new bed, and skip it when you’re only topping up an established one.

What Should You Fill Raised Beds With?

A blend of topsoil and compost is the best default raised bed soil for most home gardens. It holds water and nutrients well and gives roots something solid to grip, while lighter mixes and fillers suit special cases.

Start With the Calculator’s 60% Topsoil and 40% Compost Blend

Our calculator splits your total into 60% topsoil and 40% compost. That sits inside the common extension guidance of about one-half to two-thirds topsoil and one-third to one-half plant-based compost.

Look for loam or sandy loam topsoil. Avoid very sandy soil, which drains too fast, and heavy clay. If your topsoil feels sticky, add some coarse sand to loosen it.

Too much compost causes trouble. Compost dries out quickly and can repel water once it’s bone dry. Never fill a bed with compost alone, because it lacks the mineral part that real soil provides. Bagged products labeled “raised bed mix” are a fine shortcut if you buy bags.

Compare Mel’s Mix and Other Moisture and Aeration Ingredients

Mel’s Mix, from square foot gardening, uses equal thirds by volume: coarse vermiculite, sphagnum peat moss (or coco coir), and blended compost. It’s light and easy to work, and popular for shallow 4-by-4 beds.

Another common thirds blend uses equal parts topsoil, compost, and a drainage material such as coarse sand, perlite, pumice, or vermiculite.

Ingredient	What it does	Watch out for
Topsoil	Minerals, structure, water holding	Clay-heavy batches drain poorly
Compost	Nutrients, organic matter	Dries and repels water if overused
Coarse vermiculite	Holds moisture and air	Adds cost in big beds
Perlite	Improves drainage	Floats to the top over time
Peat moss / coco coir	Holds moisture in light mixes	Can dry out; peat has environmental concerns
Coarse sand	Loosens clay soils	Use coarse, not fine play sand

Why Straight Potting Mix or Native Clay Is Not the Default

Bagged potting mix works great in pots and seed trays. In a raised bed it dries out too quickly, and soilless mixes run low on nutrients over time. Filling a full 4-by-8 bed with it also costs a lot.

Native clay has the opposite problem. It drains slowly, stays cold in spring, and crusts in summer. If you want to use your own soil, mix in 30 to 50% compost by volume first.

Save Soil in Deep Beds With Logs, Branches, and Cardboard

Tall beds over 2 feet deep can use cheap filler in the bottom. This layered approach, related to hugelkultur, works like this:

1. Lay untreated logs and thick branches across the bottom third.
2. Add smaller sticks and leaves to fill gaps.
3. Top with at least 12 inches of your topsoil and compost blend.

Plain brown cardboard can go under the woody layer to smother grass. It blocks water until it starts to break down, so soak it well and skip it under shallow beds, where roots need direct contact with the ground. Expect the woody layer to sink as it rots, which means more topping up in the first few years.

Top Up Settled Beds Each Year

Organic matter breaks down, so beds sink a little every season. Each year, add a quarter inch to 1 inch of compost across the surface. One cubic foot of compost covers about 12 square feet at 1 inch deep.

For a 4-by-8 bed (32 square feet), a 1-inch top-up takes about 2.7 cubic feet, or three 1-cubic-foot bags. If the bed has sunk more than that, use your topsoil and compost blend for the extra and save straight compost for the top inch. Heavy compost every year can build up phosphorus and salts, so a soil test every 2 to 3 years keeps you on track.

Should You Buy Bags or Order Bulk Soil?

Bags are easiest for small beds, and bulk soil wins once your total reaches about one cubic yard. The right choice also depends on how you’ll get the soil into your yard.

Convert Your Adjusted Volume Into Whole Bags

Divide your volume with the 15% allowance by the bag size, then round up. Stores can’t sell you half a bag.

• 1 cubic foot bags: cubic feet ÷ 1
• 1.5 cubic foot bags: cubic feet ÷ 1.5
• 2 cubic foot bags: cubic feet ÷ 2

Check the label carefully. Some bags list quarts or liters. One cubic foot equals about 28.3 liters or roughly 25.7 dry quarts.

Compare Bagged Soil With Bulk at About One Cubic Yard

One cubic yard of bulk soil equals 27 cubic feet, the same as 27 one-cubic-foot bags. Buying bulk by the cubic yard is generally much cheaper than buying the same amount in bags. Delivery is usually a flat fee no matter how much you order.

That makes about one cubic yard a sensible break-even line. A single 4-by-8 bed at 12 inches already needs about 1.4 cubic yards with settling. Two or three beds push you clearly into bulk territory. A lone round bed or herb box is a bag job.

Check Delivery Access, Minimum Orders, and Storage Space

Bulk delivery has practical limits worth checking before you order:

• Access: a dump truck needs a clear path and a firm spot to drop the pile, usually a driveway.
• Minimum orders: many suppliers set a minimum, often around one cubic yard.
• Moving it: plan on a wheelbarrow and a few hours of shoveling per yard.
• Storage: cover any leftover pile with a tarp so rain doesn’t wash it away and weeds don’t sprout in it.

Bags stack neatly and keep for months, which helps if you’re filling beds over several weekends.

Buy Enough Soil for Filling and Topping Up

Getting your raised bed soil volume right comes down to accurate inside measurements and a depth that fits your crops. Multiply length, width, and depth in feet, divide by 27 for cubic yards, and add the 15% settling allowance for any new bed.

Fill most beds with a topsoil and compost blend near 60/40, and save lighter mixes like Mel’s Mix for shallow square foot beds. Deep beds can use logs and branches at the bottom to cut the amount of soil you need.

Plan for the long haul too. Set aside a few bags of compost each spring for the quarter-inch to 1-inch top-up. If you’re just getting your first beds going, our tips for starting your own garden as a beginner pair well with these numbers. For a 4-by-8 bed, that means about 37 one-cubic-foot bags to fill it and about three more each year to keep it full.

Frequently Asked Questions